The invention relates to a magnetooptical modulator having a layer of a magnetic material on a substrate. The magnetic material has an easy axis of magnetization perpendicular to the plane of the layer. The layer has a number of magnetic domain areas separated from each other by intermediate areas.
The invention furthermore relates to a method of manufacturing such a magnetooptical modulator.
Such magnetooptical modulators can be used in digital optical memories, in display screens and in optical printers. Such a magnetooptical modulator is described in DE-OS 2732282 (corresponding to U.S. Pat. Nos. 4,274,935 and 4,314,894). A plurality of magnetic domains and single domain islands, respectively, are formed in the layer of magnetic material. The magnetization of each domain can be in one of two stable directions perpendicular to the surface of the layer. The direction of magnetization of a domain corresponds to information according to a selected convention. For example, one direction of magnetization corresponds to a binary "zero" and the other direction of magnetization corresponds to a binary "one", or conversely. The directions of the domains can be observed as a light-dark distribution by exposure to polarized light, due to the magnetooptical Faraday effect.
Information can be written into the magnetic layer by applying an external magnetic field according to the desired direction of magnetization for a short period of time and at the same time applying a thermal pulse to the region of the layer to be written in. In fact, only small external magnetic fields are necessary at higher temperatures for swtiching the direction of magnetization of a domain.
Depending on the use of the magnetooptical modulator, the thermal pulse may be generated in various manners. When used as a memory, a laser beam is focused on a domain. The domain is heated by the absorbed light energy. When used purely as a light modulator, each domain is covered with a thin-film resistor which is connected electrically to crossing conductor paths. The thermal pulse can also be generated by a combination of a laser and a resistor.
Since the information content of the layer is associated with the arrangement of the magnetic areas (domains or single domain areas), the locations and the sizes of the areas must be accurately fixed. This is also necessary to reproducibly read the information. In order to obtain a large information density, the surface of the area must be as small as possible. The minimum size and the maximum density of the areas are restricted by the smallest diameter of the light spot on which the light beam can be focused on the layer, as well as by the accuracy of the deflection.
It is known that in spontaneously magnetized layers, the directions of magnetization of magnetic domains are opposite to that of adjoining domains. Adjacent domains are separated from each other by a magnetic wall. However, in the layers which are used for magnetooptical modulators and which must be switchable by means of small magnetic fields, the domains are often too large and in certain circumstances can too easily be shifted. Hence, they must be made smaller and stable as regards location by additional measures.
Fixing the magnetic single domain areas in the layer is described in DE-OS 2732282. In this publication is intermediate areas are produced by exposure to highly energetic ions or by ion implantation and an optional subsequent etching of core tracks. At the locations of these intermediate areas, there is large gradient in the magnetic properties of the layer. The intermediate areas form the walls of the single domain areas. As a result of the exposure to ions, fine structures can be obtained so that a high information density can be achieved. Moreover, the smooth surface of the magnetic layers is maintained.